Bedload Transport and Morphological Effects of High-Magnitude Floods in Small Headwater Streams - Moravskoslezské Beskydy Mts. (Czech Republic)

Abstract

Bedload transport observed during a flood in May 2010 gave rise to several forms of accumulations in small headwater basins located in the Western Flysch Carpathian Mountains, Czech Republic. We have investigated critical conditions of incipient motion of the largest boulders deposited during a c. Q 100 flood event (flood competence method). We have tested several formulas designed for high gradient streams in two small basins in the conditions of local mid-mountain relief. The results show that a flood of such a magnitude is able to transport almost all surface bed material and that bedload transport in steep headwater streams (A ≤ 1 km 2 ) is probably less selective as for the grain size than that in lower gradient gravel-bed streams. The authors discuss the importance of local basin predispostion factors in order to determine critical conditions for the onset of bedload transport. Dnový transport sedimentů zapříčinil během květnových povodní v roce 2010 vznik různých typů akumulací v malých pramenných tocích nacházejících se ve flyšových pohořích Západních Karpat. Studie se zaměřuje na určení kritických podmínek nutných pro uvedení největších klastů do pohybu, jež byly následně uloženy do akumulací během této cca Q 100 povodně (metoda účinnosti povodně). V rámci výzku-mu byly ověřeny některé rovnice vytvořené pro vysokogradientové toky na dvou malých povodích v podmínkách reliéfu hornatin. Výsledky ukazují, že povodeň takové intenzity je schopna transportovat téměř celou povrchovou vrstvu sedimentů a dnový transport je na malých povodích pravděpodobně méně velikostně selektivní než v tocích s nižším gradientem dna. Důraz byl kladen také na lokální predizpoziční faktory ovlivňující kritické podmínky pro uvedení určité velikostní frakce sedimentů do pohybu.KLÍČOVÁ SLOVA: počátek pohybu, dnový transport, pramenný tok, Moravskoslezské Beskydy.

Coarse Bed Sediments in a Headwater Channel as Indicators of Fluvial Processes and Slope-Channel Coupling: A Case Study from the Carpathian Mountains (Czech Republic)

“…Hooke (2003) reported a potentially limited connectivity for coarse sediments in the fluvial system with respect to stream power during floods. A highmagnitude flood (Q 20 ) was able to move boulders of up to 0.4 m in diameter in some upstream channelreaches (Galia, Hradecký, 2011. Connectivity in the bed-load transport of this sediment-size fraction is interrupted in downstream parts due to the decreased channel gradient, the presence of a larger space for the deposition of material between the channel and valley slopes, and a more frequent occurrence of woody debris in the channel.…”

“…Except for the prevailing erosion trends occurring due to elementary functions of mountain headwater channels, significant aggradation is identified in some reaches between 0.4 and 0.8 km, which are also affected by small woody debris jams. On the other hand, deeplyincised channel-reaches in an alluvial fan are located downstream from the 0.8 km point ( (Galia, Hradecký, 2011;. Forest cover in the studied area is close to 100%, but the socalled Wallachian colonisation resulted in extensive deforestation of the Moravian-Silesian Beskids Mts.…”

Summary of papers by Czech participants at the XVIIIth INQUA Congress in Bern

“…His method is based on the comparison of a certain grain diameter with bed surface d 50 or d 90 . Applying this approach on Malá Ráztoka stream, the calculated values of dimensionless shear stresses were from 0.02 for the 0.5 m boulders to low values of about 0.400 for 0.01 m grain diameters (Galia & Hradecký, 2011). This is further applicable to the calculations of fractional bedload transport, which the TOMSED model allows as well.…”

Development of a large wood jam in medium-high mountains: An example of the Mazák Stream, Moravskoslezské Beskydy Mts., Czechia

“…e channel is generally confined (mean bankfull width is ~ 2.5 m), but accumulations that consist of coarse sedimentary material frequently occur between the active channel and hillslopes. e width of these accumulations varies between 1 and 5 metres, and these accumulations most likely developed as consequences of past high magnitude flood events or by reactivations of older debris flow deposits by the 'firehose effect' (Šilhán, Pánek 2010;Galia, Hradecký 2011). According to Šilhán (2015) at least five floods affected the main Mazák stream during the last 30 years (1985, 1991, 1997, 2000, and 2010).…”